antiparallel orientation of the strands
Think of two lanes of a road running side by side, but with traffic flowing in opposite directions — one lane heading north, the other south. The two strands of a DNA double helix are arranged exactly like that. They lie alongside each other but point in opposite directions; biologists call this antiparallel.
Because each strand has a direction (a 5' end and a 3' end), 'antiparallel' means that where one strand's 5' end is, the other strand's 3' end sits, and vice versa. So if the top strand reads 5'-to-3' from left to right, its partner reads 5'-to-3' from right to left. Concretely, one strand might be 5'-ATGC-3' and its partner, lying head-to-tail beneath it, is 3'-TACG-5', with A facing T and G facing C across the gap. They simply cannot pair up running the same way — the geometry of the bases and sugars only fits when the strands are head-to-tail.
This opposite orientation is not a quirk; it forces real consequences. Because the strands run in opposite directions and polymerases build only 5'-to-3', the two strands cannot be copied in the same simple way during replication — one is made continuously and the other in backward pieces. Antiparallelism is the structural fact behind that asymmetry, and behind much of how the cell reads and copies its own genome.
To write the complement of a strand, you flip both the bases (A to T, G to C) and the direction: the complement of 5'-AATTC-3' is 3'-TTAAG-5', or equivalently 5'-GAATT-3' once you turn it around to read it the conventional way.
Two lanes, side by side, traffic running opposite ways.
'Antiparallel' does not mean the strands are far apart or pointing away from each other in space — they wind tightly together. It refers strictly to their chemical direction (5'-to-3'), which runs opposite on the two strands.